1. Field of the Invention
The present invention relates to an apparatus and method for determining a wavelength of electromagnetic radiation of the type emitted from a laser device, especially, though not exclusively, to such an apparatus and method for use in a wavelength locker, for example of the type that generates an error signal for controlling a wavelength of the light emitted by the laser device.
2. Brief Description of Related Developments
In a fibre-optic network, the wavelength of light used to communicate a signal is an important parameter. In particular, where Wavelength Division Multiplexing (WDM) systems are employed, different signals are communicated using respectively different wavelengths. Consequently, it is important to maintain the different wavelengths accurately in relation to components of the WDM system, for example multiplexers and demultiplexers, which add or remove wavelengths from the WDM system.
Typically, a semiconductor laser device is employed in a transmitter unit of the fibre-optic network. The wavelength of light transmitted by the laser device can be accurately controlled by altering a parameter, for example temperature, of the laser device using a closed loop feedback circuit. In this example, in order to determine whether to heat or cool the laser device, and to what extent, to lock the wavelength of the laser device to a predetermined wavelength, an error signal needs to be generated in the feedback circuit.
Known apparatus for detecting changes in the wavelength of the laser device are disclosed in U.S. Pat. No. 4,309,671, U.S. Pat. No. 6,144,025, U.S. Pat. No. 5,825,792. U.S. Pat. No. 4,309,671 discloses a laser diode, a proximal beam splitting mirror and a proximal photodiode to receive light from the proximal beam splitting mirror, a distal beam splitting mirror and a distal photodiode to receive light from the distal beam splitting mirror, and a filter is disposed between the distal beam splitting mirror and the distal photodiode. An electronic control circuit is also disclosed for stabilising the laser diode. When in use, a divergent beam is emitted by the laser diode. The proximal beam splitting mirror directs a proportion of the light incident upon the proximal beam splitting mirror onto the proximal photodiode. Similarly, light passing through the proximal beam splitting mirror is incident upon the distal beam splitting mirror, the distal beam splitting mirror directing a proportion of the light incident upon the distal beam splitting mirror onto the distal photodiode. The filter has a positive transmission gradient versus wavelength characteristic, so that the light passing through it is selectively attenuated depending on its wavelength. The two photodiodes are coupled to an amplifier whereby their ratio can be utilised as a measurement of the wavelength of the light emitted by the laser, and variations in that ratio (indicative of a variation in wavelength of emitted light) are used in feedback loop to control the laser diode. Usually, since such variations in the wavelength of light emitted by the laser diode are due to variations in temperature, the feedback loop is used to adjust the temperature of the laser diode.
U.S. Pat. No. 5,825,792 discloses an apparatus comprising a lens, a Fabry-Perot etalon and two photodiodes, the apparatus being copackaged with a semiconductor laser in an industry standard package known as a “butterfly” package. The etalon splits light emitted by the semiconductor laser and propagates the light over multiple paths of different lengths before recombination. Respective phases are accumulated over the multiple paths, the phases accumulated being wavelength-dependent. Consequently, the result of the recombination also depends upon wavelength. Again, the two photodiodes are coupled to an amplifier whereby their ratio can be utilised as a measurement of the wavelength of the light emitted by the laser, and variations in that ratio (indicative of a variation in wavelength of emitted light) are used in feedback loop to control the laser diode. By using an etalon, in which light transmission is caused by interference between light paths, the transmission characteristic is cyclical. Although, therefore, the same ratio between the outputs of the photodiodes will occur at different wavelengths whose transmission levels are the same, once the correct wavelength has been found, this apparatus will maintain the laser at that wavelength in the manner described above.
The dimensions of the etalon depend upon a required resolving power, R, of the etalon; the resolving power is a measure of a minimum change of wavelength that can be detected. The resolving power, R, of the etalon is given by the following equation:   R  =      F    ⁢                  2        ⁢                                  ⁢        n        ⁢                                  ⁢        d                    λ        o            where:                F is the coefficient of finesse,        n is the refractive index of the etalon,        d is the thickness of the etalon, and        λo is the wavelength of operation.        
As a practical example, in order to monitor a 100 GHz or 50 GHz channel spacing, at least one dimension of the etalon has to be approximately 1 mm or approximately 2 mm, respectively.
U.S. Pat. No. 6,144,025 discloses a laser diode coupled to a first optical fibre. When in use, light emitted by the laser diode propagates through the first optical fibre, a lens, a cut filter, after which the light is incident upon a beam splitter. A first photodiode is located on a first side of the beam splitter and a second photodiode is located on a second side of the beam splitter. An optical band-pass filter is disposed in-line between the beam splitter and the first photodiode. A proportion of the light incident upon the beam splitter is directed towards the first photodiode. A first proportion of the light directed towards the first photodiode is permitted to pass through to the first photodiode and a second proportion of the light directed towards the first photodiode is reflected by the optical band-pass filter to the second photodiode via the beam splitter. A certain proportion of the light incident upon the beam splitter via the cut filter is permitted to pass directly through the beam splitter to a lens that focuses the transmitted light into a second optical fibre.
In the apparatus of U.S. Pat. No. 6,144,025 the two photodiodes are coupled to an output ratio calculator, whose output is coupled to a wavelength controller in a feedback path to control the laser diode, in a similar manner to that described above. As an alternative to the optical band-pass filter, there can be used an interference filter to change the wavelength of light transmitted by the filter to the first photodiode.
As optical communication systems become more complicated, and it becomes desirable to utilise more separate wavelength channels, lasers that are not single wavelength are becoming more prevalent, since it is easier to tune them to operate at another wavelength than to replace them by another laser operating at the different wavelength. These tunable lasers often have a tunable range of approximately 30–40 nm. Furthermore, although the ITU presently requires a spacing of at least 0.8 nm between adjacent signalling channels, as demand for channels grows, it will be necessary to reduce the spacing further, so that the resolution of the wavelength detector and the laser locker will need to be higher than hitherto.